Important First Sentences

"It was the best of times, it was the worst of times..." - A Tale of Two Cities
"It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife." - Pride and Prejudice
"There was a boy called Eustace Clarence Scrubb, and he almost deserved it." - The Voyage of the Dawn Treader

The first sentences of books are important. They orient us. They make us want to know more. They are the beginning of our understanding what the author means to tell us. Writers spend hours pondering the opening lines because they know how important they are.
Perhaps the greatest example of this is the Bible. Its opening sentence is, "In the beginning God created the heavens and the earth." Few sentences are more important. It helps us understand ourselves and our world.
Wisdom begins with grasping the fact that there is a God.
Unfortunately, many people have been educated in a worldview that excludes the idea of God. They have been told that modern science has "proven" that God does not exist. When you try to share the gospel with them they will cut you off with a wave of the hand saying, "I am an atheist."
Sharing the gospel with someone like that can feel hopeless.
Where do we begin when they reject the very first sentence of the Bible?

Some things to remember

First of all, do not be intimidated. Remember: you know God! While they may have many fine sounding arguments and secular education tends to be antagonistic to faith, you have truth on your side. God does exist. You have encountered him. You have met the risen Saviour. Think of it this way: would you be intimidated by someone who "didn't believe in the Alps" if you had been there? Of course not. Remember: You know God. As frustrating as it may be to explain what you know to someone who claims not to believe, the truth is that God exists. Let that be the foundation that you stand on.
Second, for anyone to come to faith in Christ takes a miracle. Moving from unbelief to accepting the gospel is impossible without the work of the Holy Spirit. It isn't your words or fine-sounding arguments that will open the person's eyes. The Holy Spirit will take what you say and use it in the person's life. So your words do not have to be perfect. You don't have answer every objection. You cannot argue someone into the Kingdom of God. The Holy Spirit needs to do the work.

A Simple Starting Point

Sometimes I will begin by saying something like this, "That's really interesting. You know, I've met a lot people who claim to be atheists but then I found out they were really are something else. Can I ask you a question?"
They always say yes.
Then I pull out a piece of paper (a napkin will do just fine) and put it in front of me. "Let's say that this piece of paper represents all that there is to know. That's a lot of stuff."

The knowledge Michel has does not include God.

Then I draw a line across the paper. "You're a smart person, but I am sure you will agree that you don't know everything. So let's say that this part of the paper represents everything that you do know." I write their name on that side of the paper.
Then I draw a small circle on the other half of the paper and label it 'God'.
"Isn't it possible that God exists, but is outside of your current range of knowledge?"
Everyone I have ever asked has agreed.
"Since you say that is possible, I think it would be more accurate to call your position that of skeptic, agnostic or doubter." They nod their head in agreement.

Why is this approach important? 

Those that call themselves atheists are closed to the idea of God. They are firmly holding to an intellectual position and have all of their defences up. Often they are exuding a kind of arrogance that won't allow meaningful conversation. They will rebuff any argument you make or testimony you give regarding the existence of God.
Spiritually their position is that of a person in a stormy ocean desperately clinging onto a small rocky island. Left on that island they will die. There is a rescue boat nearby, but in their fear they won't venture into the sea. They will never be rescued. Something needs to be done.
Pushing them off the island back into the water is only way to get them to the safety of the ship. You need to set them adrift. The way to do that is to get them to admit to the possibility that God exists. Peeling off their self-label of "atheist" and replacing it with "agnostic" or "skeptic" is a small but critical first step. It pushes them back into the water so it is possible to find the rescue ship.
Second, it opens the door to the idea that you (or some other Christian) might know something that they don't know. This is important because it moves them from a position of defence to inquiry. And often from arrogance to humility. It can help them listen to philosophical arguments for the existence of God. It can help them be willing to explore what the Bible says. It can cause them to listen to your experience of knowing God.
The Lord can use this to set them searching for what to them is now "the unknown God."

Where to next?

From here the conversation can go a million different directions. Sometimes we can share one of the philosophical arguments for the existence of God. More often it will be more natural to share a brief testimony from our experience of knowing God. Your testimony is evidence that you know or have experienced something that your friend has not yet experienced. They have already agreed they do not know everything.
Occasionally the conversation will drift away toward more mundane topics, but that is okay. We have accomplished much by moving the person away from atheism and towards a search for God. The Holy Spirit will use that. So pray that the Lord will reveal open the eyes of your friend.